Until FCPX 10.1 and Mavericks Sony’s XAVC “S” was impossible to deal with on my fully spec 27″ iMac with Thunderbolt. The first thing you should note is that 4K comes in two sizes, 17:9 which is known as true 4K (4096 x 2160) and 16:9 which is 3840 x 2160.
It’s not a major problem if you can’t decide what to choose as you can easily change the size in FCPX 10.1.
This is a 16:9 XAVC “S” file on a true 4K timeline (17:9) and as you can see you get two black borders either side of the picture, to sort this scroll down to the Spatial Conform section.
If you have a Sony FDR-AX1 you will find XAVC “S” which has an MPEG 4 wrapper, at 4K you only get the option of 3840 x 2160 which is 16:9. The good news is that FCPX 10.1 has nailed the ingest of this complicated codec and takes no longer than XAVC to import.
You will get three options, Fit…Fill…None, use the “Fill” option and you will lose your two black borders. If you have Full HD footage (1920 x 1080) and you choose “None”under Spatial Conform you can fit four HD shots into one 4K page.
Interestingly I decided to fit a 1920 x 1080 picture into a 4096 4K timeline to see what the quality loss would be and the simple answer is very little, I was surprised at this but will give me great confidence in the future if I have to mix and match HD and 4K material.
FCPX 10.1 with Mavericks is a game changer for 4K in general, Sony’s XAVC “S” did not work with FCPX 10.0.9 and Mountain Lion. Apple have done their homework with Mavericks but combine this with FCPX 10.1 and you get a very slick, responsive editing system add that to a new MacPro and 4K becomes even more fantastic.